This application is related generally to temperature sensing and, more specifically, to methods and apparatus for sensing a temperature and generating a digital output proportional to the sensed temperature. The preferred embodiments will be described with reference to example circuits and with reference to example method steps. However, it is to be understood that the claimed embodiments may be implemented in a wide variety of forms, components and method steps and therefore is not limited to the example preferred embodiments. As an example, other equivalent circuits and steps may be used to sense a temperature and generate a digital output proportional to the sensed temperature, and the applications may include differential temperature sensing.
FIG. 1 is a functional block diagram illustrating a basic temperature sensing circuit 10 in accordance with the prior art. As shown, in general, the temperature is determined by comparing the output of a temperature dependent voltage generating circuit 12 with a substantially temperature independent reference voltage generating circuit 14. More particularly, a complementary to absolute temperature (CTAT) circuit 16 generates a CTAT voltage signal V1 and a fixed voltage generating circuit 18 generates a reference voltage V2. A comparator circuit 20 is included in the basic temperature sensing circuit 10 together with an offset circuit 22 and an error correction circuit 24.
As shown in FIG. 2, in general, the CTAT voltage signal V1 varies substantially linearly with changes in temperature and, more particularly, decreases for temperature increases. The negative slope of the CTAT voltage signal V1 is typically about 200-300 mV over about 150° C. or about 1.6 mV/° C. In the illustrated prior art example, the temperature circuit 10 is operational in the box bounded by v1, v2, t1, and t2.
In operation and with reference to both FIGS. 1 and 2, the basic prior art circuit 10 generates a temperature output signal 26 based on an amount of offset needed to be added to the reference voltage V2 so that a sum of the offset voltage V3 and the reference voltage V2 at a first input 28 to the comparator circuit 20 matches the level of the CTAT voltage signal V1 at the second input 30 to the comparator circuit 20. Essentially, the output signal 26 is proportional to the amount of offset needed to be added to the reference voltage V2. At a nominal temperature tnom in the example shown no offset is needed so the reference voltage V2 is presented directly at the first input 28 and the temperature output signal 26 is proportional to an offset value of zero. At the low temperature extreme t1 in the example shown the offset needed to be added to the reference voltage V2 is (v2−V2) and the output signal 26 generated is proportional to (v2−V2). At the high temperature extreme t2 in the example shown the offset needed to be added to the reference voltage V2 is (v1−V2) and the output signal 26 generated is proportional to (v1−V2).
Basic temperature sending circuits of the type described above perform adequately for some applications. However, the range of the output V1 of the temperature dependent circuit 12 such as between v1 and v2 in the example is relatively small making the circuit generally sensitive to changes such as drifting etc. in the temperature independent reference voltage circuit. Small changes in V2 produce inaccurate results.
Overall, for a temperature range of interest, it is desired to maximize the change of the temperature dependent voltage signal V1 relative to the temperature independent reference voltage V2. Graphically it is desirable to maximize the angle of incidence A between the temperature dependent voltage signal V1 and the temperature independent reference voltage V2. However, the range of the output V1 of the temperature dependent circuit 12 such as between v1 and v2 results in a relatively small angle making the temperature sensor circuit sensitive.
In addition, the output V2 from the reference voltage circuit 14 is in practice temperature dependent. In that regard, typically, the slope of the output V2 is slightly negative. This further reduces the angle of incidence A, making the circuit less precise.